This example describes how to setup a new Foundation Library in MicroEJ SDK and how to distribute it through a MicroEJ Platform.

A Foundation Library is a library that provides core runtime APIs or hardware-dependent functionality. It is often connected to underlying C low-level APIs.

This example has been tested on:

• MicroEJ SDK 5.4.1

• With a MicroEJ 5 Demonstration Platform imported into the MicroEJ repository that contains:

  • EDC-1.3
  • BON-1.4
  • SP-2.0

  See https://developer.microej.com/getting-started-sdk.html.

• Basic knowledge about Java and C programming.
• Basic knowledge about MicroEJ (Platform build and Standalone Application launch).

A Foundation Library is composed of

• A MicroEJ API project: contains API skeletons for compilation purpose.
• A MicroEJ Implementation project: contains the runtime code executed by the Platform and Low Level C header files.
• C code: contains the implementation of native methods linked to the C project.
• Java Mock-up project: contains the implementation of native methods for simulation.

LLAPI

• Select File > New > Module Project
  • Set the project settings.
    • Project Name: mylib-api
    • Organization: com.mycompany.api
    • Module: mylib
    • Revision: 1.0.0
  • Select microej-javaapi skeleton.
  • Click on Finish.
• Select File > New > Class
  • Source folder: mylib-api/src/main/java.
  • Package: com.mycompany.
  • Name: MyLib.
• Copy and paste the following code into this class:

package com.mycompany;

/**
 * My Foundation Library.
 */
public class MyLib {
    /**
     * Computes the factorial of an integer.
     *
     * @param number
     *            a positive integer
     *
     * @return the factorial of number.
     */
    public static int factorial(int number) {
        throw new RuntimeException();
    }
}

This class defines a factorial API. The method content is filled with throw a new RuntimeException just for successful compilation.

Right-click on mylib-api project and select Build Module. After a successful build, the project build directory target~/artifacts contains:

• Jar file (mylib.jar), that will be used by an Application.
• Rip file (mylib.rip), that will be embedded into a Platform.

• Unzip mylib.rip and copy all the files of the content directory into the dropins directory of the [platform]-configuration project.
• Rebuild the Platform.

• Select File > New > Module Project
  • Set the project settings.
    • Project Name: mylib-impl
    • Organization: com.mycompany.impl
    • Module: mylib-impl
    • Revision: 1.0.0
  • Select microej-javaimpl skeleton.
  • Click on Finish.
• Open mylib-impl/module.ivy
  • replace microej.lib.name="mylib-impl-1.0" with microej.lib.name="mylib-1.0"
  • Save the file.
• Select File > New > Class .
  • Source folder: mylib-impl/src/main/java.
  • Package: com.mycompany
  • Name : MyLib
• Copy and paste the following code into this class:

package com.mycompany;

@SuppressWarnings({ "javadoc", "nls" })
public class MyLib {

    public static int factorial(int number) {
        if (number < 0) {
            throw new IllegalArgumentException("Factorial cannot be negative");
        }
        return nativeFactorial(number);
    }

    public native static int nativeFactorial(int number);
}

This class defines the factorial implementation. It first checks the argument validity and then redirects to a native method for speed consideration.

• Create a new file named include/LLMYLIB_impl.h into the content directory of the implementation project.
• Copy and paste the following code into this file:

#ifndef LLMYLIB_IMPL
#define LLMYLIB_IMPL

/**
 * @file
 * @brief MicroEJ factorial Low Level API
 * @author My Company
 * @version 1.0.0
 */

#include <stdint.h>

#ifdef __cplusplus
extern "C" {
#endif

#define LLMYLIB_IMPL_factorial Java_com_mycompany_MyLib_nativeFactorial

/*
 * Returns the factorial
 */
uint32_t LLMYLIB_IMPL_factorial(uint32_t number);

#ifdef __cplusplus
}
#endif
#endif

This file defines the factorial C prototype. The com_mycompany_MyLib part is the fully qualified name of the MyLib class created previously where all . are replaced by _.

The #define statement allows to separate the Java part and the C part. This is called the Low Level API of the Foundation Library. If the fully qualified name of the Java native method is updated, the C implementation code do not need to be updated.

Right-click on mylib-impl project and select Build Module. After a successful build, the project build directory target~/artifacts contains:

• Rip file (mylib-impl.rip), that will be embedded into a Platform.

• Unzip mylib-impl.rip and copy all the files of the content directory into the dropins directory of the [platform]-configuration project.
• Rebuild the Platform.

• Create a new project File > New > Standalone Application Project
  • Set the project settings.
    • Project Name: mylib-test
    • Organization: com.mycompany.test
    • Module: mylib-test
    • Revision: 1.0.0
• Open module.ivy
  • Add the dependency <dependency org="com.mycompany.api" name="mylib" rev="1.0.0" />
• Open file Main.java
  • Source folder: mylib-test/src.
  • Package: com.mycompany
  • Class Name: TestMyLib
• Copy and paste the following code into this class:

package com.mycompany.test;

public class Main {

   public static void main(String[] args) {
      System.out.println("(5!)=" + MyLib.factorial(5));
   }
}

This class defined a main entry point that prints the result of 5!.

• Right-click on mylib-test project and select Run As > MicroEJ Application.

The application is started. After a few seconds, the following trace shall appear in the console view:

Exception in thread "main" java.lang.UnsatisfiedLinkError: No HIL client implementor found (timeout)
    at java.lang.Throwable.fillInStackTrace(Throwable.java:79)
    at java.lang.Throwable.<init>(Throwable.java:30)
    at java.lang.Error.<init>(Error.java:10)
    at java.lang.LinkageError.<init>(LinkageError.java:10)
    at java.lang.UnsatisfiedLinkError.<init>(UnsatisfiedLinkError.java:10)
    at com.mycompany.MyLib.factorial(MyLib.java:15)
    at com.mycompany.TestMyLib.main(TestMyLib.java:5)
    at java.lang.MainThread.run(Thread.java:836)
    at java.lang.Thread.runWrapper(Thread.java:372)

This is the normal behavior because nativeFactorial native method is currently not implemented (see below). The HIL engine (Hardware In the Loop) did not find a Platform Mock-up implementing the native method.

To each MicroEJ native method is associated a Java Mock-up method that implements the simulated behavior. A Mock-up project is a standard Java project (J2SE).

• Select File > New > Module Project
  • Set the project settings.
    • Project Name: mylib-mock
    • Organization: com.mycompany.mock
    • Module: mylib-mock
    • Revision: 1.0.0
  • Select microej-mock skeleton.
  • Click on Finish.
• Select File > New > Class
  • Source folder: mylib-mock/src/main/java.
  • Package: com.mycompany
  • Class Name: MyLib
• Copy and paste the following code into this class:

package com.mycompany;

public class MyLib {

    public static int nativeFactorial(int number) {
        if (number == 0) {
            return 1;
        }
        int fact = 1; // this will be the result
        for (int i = 1; i <= number; i++) {
            fact *= i;
        }
        return fact;
    }
}

This class defines the implementation nativeFactorial method on Simulator. The Mock-up method has the same prototype than the implementation one, except the native modifier. The HIL engine will link the native method to the Mock-up method.

• Right-click on the mylib-mock project and select Build Module.

After a successful build, the project build directory target~/artifacts contains:

• Rip file (mylib-mock.rip), that will be embedded into a Platform.

• Unzip mylib-mock.rip and copy all the files of the content directory into the dropins directory of the [platform]-configuration project.
• Rebuild the Platform.

• Right-click on mylib-test project and select Run As > MicroEJ Application.

The following trace shall appear in the console view:

=============== [ Initialization Stage ] ===============
=============== [ Launching on Simulator ] ===============
(5!)=120
=============== [ Completed Successfully ] ===============

• Duplicate the Simulation launcher
  • Go to Run > Run Configuration…
  • Select mylib-test TestMyLib launcher
  • Right-Click and select Duplicate
  • In Execution tab, select Execute on Device"
• Click on Run

The file microejapp.o is generated to a well known location for the C project.

• Open the Platform C project into the C IDE
• Compile and link the project

Please consult the documentation of the imported Platform for more details on the proceedings.

A similar linker error than the one below should appear in the C IDE console view:

Undefined symbol Java_com_mycompany_MyLib_nativeFactorial (referred from microejapp.o).

This is the normal behavior because the symbol Java_com_mycompany_MyLib_nativeFactorial is currently not implemented in C code. The third-party linker did not find an object file implementing the native function.

• In the C project, create a new File called LLMYLIB_impl.c
• Add the C file to the compilation objects by adding it to the C Project configuration
• Copy and paste the following code to the file:

#include "LLMYLIB_impl.h"
#include "sni.h"

/**
 * @file
 * @brief MicroEJ factorial low level API (the implementation does not support unsigned integer overflow)
 * @author My Company
 * @version 1.0.0
 */
uint32_t LLMYLIB_IMPL_factorial(uint32_t number)
{
    if (number == 0) {
        return 1;
    }
    uint32_t fact = 1; // this will be the result
    for (uint32_t i = 1; i <= number; i++) {
        fact *= i;
    }
    return fact;
}

This file defines a basic C implementation of the nativeFactorial function.

• Link the C Project.

The link shall produce the executable file.

• Program the executable file on the device.

The following trace shall appear on the standard output:

VM START
(5!)=120
VM END (exit code = 0)

• Communication mechanisms from Java to C: Example-Standalone-Java-C-Interface
• Simulation mock specification
• Generate a mock with an UI: Mock-Get-Started